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Objective: To review and systematize all epidemiological studies of seasonal affective disorder (SAD). Method: The relevant papers were identified by searches in Medline, Excerpta Medica, PsychLIT and other databases. The primary reports were reviewed for additional citations. The studies were classified into retrospective and prospective population surveys, surveys of patient populations and studies of seasonal variations in psychiatric illnesses other than mood disorders. Results: The prevalence estimates of SAD across 20 retrospective studies varied from 0% to 9.7%. All prospective population studies, except one, find seasonal variations in mood, depressive symptoms usually peaking in winter. SAD was more prevalent at higher northern latitudes, but the prevalence varied across ethnic groups. SAD has also been identified in children and adolescents. Seasonal exacerbations and remissions are not limited to mood disorders, it has also been found in bulimia nervosa, anxiety disorders and other psychiatric illnesses. Conclusions: The actuality of seasonal variation in mood has been documented thoroughly by both retrospective and prospective studies. In the general population, depressive symptoms peak in winter, and the most extreme form of this disposition, SAD, appears to be a relatively common disorder.